The invention relates to an anti-lock brake system (ABS) for a road vehicle which is also equipped with a wheel slip control system (ASR) which operates on the principle of decelerating a vehicle wheel tending to spin, by activating its wheel brake, so that wheel slip remains held within a defined range of values compatible with adequate directional stability. The brake system has a hydraulic dual-circuit brake system having two static brake circuits, one brake circuit being allocated to the undriven wheels of the vehicle and the other brake circuit being allocated to the driven wheels of the vehicle. A brake unit is provided for supplying brake pressure to the two brake circuits and has a primary and a secondary output-pressure space, one of which is allocated to each of the brake circuits respectively, and from which brake pressures can be applied to the brake circuits after being built up by pedal-force-controlled displacement of a piston means movably delimiting the output pressure spaces. A high-pressure pump is provided for each of the two brake circuits to feed brake fluid into the output pressure spaces of the brake unit in control phases of the ABS system for the purpose of preventing the brake circuits from becoming empty as well as to obtain a warning as to the activation of the ABS system. The high-pressure pump for the brake circuit of the driven vehicle wheels also provides an auxiliary pressure source for the ASR system to supply brake pressure to the wheel brake(s) of the driven wheels subjected to wheel slip under control of brake pressure control valves individually allocated to the wheel brakes control. The brake circuit of the driven vehicle wheels is connected and disconnected to the secondary output-pressure space, which is separated with respect to the primary output-pressure space of the brake unit by a floating piston, under control of output signals from an electronic control unit by an ASR Control valve for the duration of control cycles of the ASR system. The ASR control valve assumes a throughflow position during normal braking mode and braking mode subjected to an anti-lock control wherein the brake circuit of the driven vehicle wheels is connected to the secondary output-pressure space of the brake unit.
An ABS control combined with an ASR control is known from German Patent DE No. 31 37 287.
The known ABS operates on the so-called feedback principle, wherein during pressure reduction control phases of the anti-lock control, the brake fluid discharged from the wheel brake(s), subjected to ABS control, is pumped back into the output pressure space of a brake unit designed as a tandem master cylinder by return pumps individually allocated to the respective brake circuits. Here it is presupposed that the brake circuits are closed brake circuits. In pressure build-up phases of the wheel slip control, the pressure loading of the wheel brake(s), subjected to known ABS and ASR, is effected by feeding in pressure from a pressure accumulator provided as an auxiliary pressure source of the ASR under control of a function control valve. In order to achieve as simple an overall structure as possible of the two combined control systems, i.e., to save on hydraulic functional components to the greatest extent possible, the ABS return pump allocated to the brake circuit of the driven vehicle wheels (normally the rear axle brake circuit) is utilized as accumulator charging pump for the pressure accumulator provided as pressure source for the wheel slip control. For this purpose, a 3/2-way solenoid valve is used both as ASR control valve and as accumulator charging valve and in its neutral position allows brake pressure to be fed out of the brake unit into the wheel brakes of the brake circuit of the driven vehicle wheels. In this neutral position, the pressure accumulator is shut off from the main brake line branching towards the wheel brakes. In the excited position of this valve, the brake unit is shut off from the main brake line of the brake circuit of the driven vehicle wheels while the pressure accumulator is then connected to this main brake line. In this excited position of the function control valve, the pressure accumulator can also be charged by a return pump of the ABS. The brake circuit of the driven vehicle wheels is also closed in ASR mode, in the sense that brake fluid discharged in pressure reduction phases of the wheel slip control from the wheel brake(s) subjected to ASR control is pumped back into the pressure accumulator. However, to ensure that this pressure accumulator cannot be exhausted by a large number of successive control cycles of the wheel slip control, since each pressure build-up and pressure reduction cycle of the wheel slip control is associated with a reduction in the contents of the accumulator which, though small, is not negligible (the pressure drop per control cycle being determined essentially by the opening pressure of inlet non-return valves of the return pumps a separate charging circuit must additionally be provided for the pressure accumulator, by which circuit the pressure in the pressure accumulator can, from time to time (i.e., when the accumulator pressure has fallen below a limiting value monitored by a pressure switch), be raised again to a higher initial value. For this purpose, a further accumulator solenoid charging valve is provided to allow brake fluid to pass from a brake fluid reserve to the return pump and be pumped into the accumulator by the return pump.
Although in this ABS combined with a wheel slip control, a high-pressure pump which is otherwise provided for the charging of the pressure accumulator, is eliminated, the overall expenditure required for electrohydraulic peripherals (the additional accumulator charging valve, the pressure switch, the high-pressure accumulator and a buffer accumulator, which is connected upstream of the return pump) is by no means inconsiderable.
It is therefore the object of the invention to specify a structure for an anti-lock brake system of the type mentioned at the outset combined with a wheel slip control system at low cost in terms of equipment without, in principle, impairing the functioning of the two control systems.
This object is achieved at the brake circuit of the undriven wheels of the vehicle, operating on the feedback principle during ABS system control, wherein during pressure reduction phases of the anti-lock control, brake fluid is discharged from at least one of the Wheel brakes of the undriven vehicle wheels and is pumped back into the primary output-pressure space of the brake unit. The brake circuit of the driven vehicle wheels operates on the discharge principle during ABS system control wherein, during brake pressure reduction phases of the anti-lock control, brake fluid is discharged at one of the wheel brakes of the driven vehicle wheels and is channelled off towards a brake fluid reservoir of the brake system and wherein for a duration of the activation of the ABS system, brake fluid is pumped out of the reservoir into the secondary output-pressure space by a high-pressure pump. Inlet pressure control valves and outlet valves are provided for each wheel brake of the driven vehicle wheels. The inlet and outlet valves are separately controllable by output signals from an electronic control unit. A neutral position of the inlet valves is their throughflow position, and an excited position of the inlet valves is their blocking position. A neutral position of the outlet valves is their blocking position, and an excited position is their throughflow position. If a signal characteristic of activation of the ASR control is still present at the beginning of braking, the electronic control generates output signals for a specifiable retardation period t.sub.r, the duration of which is between 100 and 300 ms; and wherein the output signals hold the ASR control valve in its blocking position, switches the inlet valves back into their neutral position, sets the outlet valves into their excited position and switches off the drive of the high-pressure pump.
According to this the ABS return pump allocated to the brake circuit of the driven vehicle wheels is utilized directly as auxiliary pressure source for the ASR, from which pressure source, in brake pressure build-up phases of the wheel slip control, brake fluid is conveyed directly into the wheel brake(s) subjected to control. A function control a 2/2-way solenoid valve of simple design is utilized when the wheel slip control comes in to shut off the output pressure space allocated to the rear axle brake circuit of the tandem master cylinder from the wheel brakes. Furthermore, the ABS is designed in such a way that at the undriven wheels (the front axle), it operates on the feedback principle, but at the driven vehicle wheels it operates on the discharge principle, according to which, in brake pressure reduction phases of the anti-lock control, brake fluid is discharged out of the wheel brake(s) subjected to control, directly into the brake fluid reservoir through appropriate control of the brake pressure control valves provided for brake control at the driven vehicle wheels. During the braking operation subjected to anti-lock control, brake fluid is fed into the secondary output-pressure space of the brake unit, which space is limited in movable fashion by the floating piston of the master cylinder due to the high-pressure pump allocated to the brake circuit of the driven vehicle wheels to thereby prevent the possibility of said secondary output-pressure space being emptied. If the ASR is still activated at the beginning of braking, e.g. in the sense of a brake pressure build-up phase, and, for a retardation period t.sub.r which begins, for example, with the onset of the brake light output signal, the ABS and ASR control unit generates output signals by which: the ASR control valve is set to its blocking position; the brake pressure control valves of the wheel brakes of the brake circuit of the driven vehicle wheels are set to their brake pressure reduction position; and the drive of the high-pressure pump is switched off. Within the retardation period t.sub.r, which is fixed or variable and is designed to have a duration between 100 and 300 ms, preferably about 200 ms, the brake pressure possibly still prevailing in the wheel brakes is thereby reduced at least as far as possible and only then are the valves switched back into their neutral positions permitting the build up of brake pressure by brake actuation. With the switching over of the brake pressure control valves into their pressure reduction position, the pump utilized prior to this as high-pressure source for the rear axle brake circuit is switched off.
The minimum advantages obtained by this design of the ABS and ASR systems and the configuration of the function control provided for the two control systems, as compared with the known anti-lock and wheel slip control systems, are as follows.
A considerable constructional simplification is achieved, since a pressure accumulator for the ASR is no longer required and hence components, such as an additional accumulator charging valve and a pressure switch, otherwise necessary for recharging it outside control cycles of the ASR are also dispensed with. Likewise, a buffer accumulator for the rear axle brake circuit is not utilized. The reduction in the technical expenditure required for implementing the two control systems is considerable.
Admittedly, it must be accepted that in the control system according to the invention, the reactive effect clearly recognizable (possibly pulsating) and perceptible at the brake pedal in the case of a response of the ABS in causing withdrawal of the brake pedal in the direction of its neutral position, is somewhat different to that in known control systems. This arises from the fact that when the ABS responds, brake fluid is fed into the output pressure space allocated to the rear axle brake circuit of the brake unit until the floating piston, limiting the output pressure space of the brake unit, has reached a position near to its neutral position, i.e., a position approximately corresponding to the latter. Here, the quantity of brake fluid conveyed back exclusively into that output pressure space of the brake unit which is allocated to the undriven vehicle wheels, is only that quantity required for pressure reduction in the front wheel brakes subjected to control. This is to ensure that system can still develop braking forces, which permit the utilization of a maximum possible tangential force corresponding to the adhesion coefficient between the roadway and the wheels.
However, this slight functional difference of the combined control system according to the invention can be regarded as insignificant, since in all cases a reliable warning of the response of the ABS is communicated to the driver and this warning is also not too disturbing even if when the ABS responds an essentially constant brake fluid quantity makes a constant contribution to the pedal warning by being fed into that output pressure space of the brake unit which is allocated to the rear axle brake circuit at the same time as a variable brake fluid quantity is conveyed back into that output pressure space of the brake unit which is allocated to the brake circuit of the undriven vehicle wheels (normally, the front axle brake circuit). The quantity of brake fluid is correlated with the braking force still utilizable at this brake circuit. By virtue of the pressure reduction phase in the wheel brakes of the driven vehicle wheels, which pressure reduction phase is triggered in the case of a very rapid transition from a wheel slip-controlled acceleration mode into a braking mode, the possibility of the occurrence of jolting reactive effects on the secondary piston limiting the secondary output-pressure space of the brake unit due to a residual pressure, which can be relatively high (resulting from a wheel slip control and still present at the start of a braking operation) is reliably eliminated. Such reactive effects could lead to a corresponding jolting and unpleasant reactive effect on the brake pedal and, in unfavorable cases, even cause damage to a central valve provided on the secondary piston of the brake unit. This control valve, in the neutral position of the secondary piston, frees the compensating flow path with the conclusion of a braking operation. The pressure compensation between the brake circuit and the brake fluid reservoir is effected and which, in the case of controlled braking when the secondary piston, as a consequence or the feeding of brake pressure into the secondary output-pressure space of the brake unit, reaches it near-neutral position, permits compensating flows from the secondary output-pressure space to the brake fluid reservoir. This results in the pressure prevailing in the secondary output-pressure space of the brake unit being held at the expected value input by driver actuation of the brake pedal.
By virtue of the fact that the inlet valves of the wheel brakes of the brake circuit of the driven vehicle wheels are held open for the duration of the retardation period t.sub.r while at the same time the outlet valves of these wheel brakes are likewise open, the possibility that due to continued running of the high-pressure pump allocated to the brake circuit of the driven vehicle wheels, an excessive pressure will build up in the main brake line of said circuit, which could lead to damage to the pump itself and/or to the main brake line, is avoided.
In the case of a transition from a wheel slip-controlled acceleration mode of the vehicle to a braking operation of the latter, the braking effect in the brake circuit of the undriven vehicle wheels (normally, the front wheels) comes in without delay, while the braking effect in the brake circuit of the driven vehicle wheels (normally, the rear wheels) comes in only after the said retardation period. This is in no way a disadvantage with respect to the dynamic stability of the vehicle. Nevertheless, it is advantageous, with a view to optimum vehicle deceleration, if the brake force friction which can be generated by the wheel brakes allocated to the driven vehicle wheels can take effect as early as possible during a braking operation.
Having the features of an anti-lock brake system, wherein the electronic control unit generates the output signals during the retardation period t.sub.r for holding the ASR control valve in its blocking position in response to one of a vehicle brake light switch output signal and output signals of wheel speed sensors allocated to the driven vehicle wheels (the speed sensor output signals are characteristic of a dynamic behavior of the driven vehicle wheels), specify criteria and measures which are favorable and makes possible an early detection of the transition, from a vehicle acceleration mode and to wheel slip control to a vehicle braking mode and which, if required, is controlled.
The manner of proportioning the .duration of the retardation period t.sub.r is by having the retardation period t.sub.r be in a monotonic interrelationship with a duration t.sub.a of the pressure build-up phase of the wheel slip control preceding the braking operation, and preferably having it increase exponentially with the duration t.sub.a of the pressure build-up phase. This can be implemented without significant additional technical expenditure by an appropriate design of the electronic ABS and ASR control unit. This also allows for reducing this period to a minimum necessary to effectively eliminate the stated risk of damage, i.e., to achieve a sufficient brake pressure reduction following which, and only then, brake pressure can be built up again.
The high pressure pump of the ABS system and the high-pressure pump of the ASR system from which, in control phases of the latter, the brake-pressure loading of the wheel brake(s) subjected to control of the brake circuit of the driven vehicle wheels is effected, are designed as reciprocating pumps having a common eccentric drive, and wherein a bypass flowpath in parallel connection to the ASR control valve and having a pressure relief valve is provided.
The pump assembly can, on the one hand, be implemented in terms of construction in the same way as the return pump assembly of an ABS operating on the feedback principle, both at the brake circuit of the undriven vehicle wheels and at the brake circuit of the driven vehicle wheels. On the other hand, it is also ensured that an excessively high pressure cannot be applied to the brake circuit of the driven vehicle wheels.
Also by having a precharging pump, by which brake fluid can be conveyed out of the reservoir of the brake system into the pump chamber of the high-pressure pump of the brake circuit of the driven vehicle wheels, and wherein the high-pressure pump allocated to the brake circuit of the driven vehicle wheels is designed as a self-acting reciprocating suction pump, alternative measures for supplying the inlet of the high-pressure pump utilized as auxiliary pressure source for the ASR, can be made technically more simple.
A particularly simple construction of the pump assembly is achievable by having the reciprocating pump of the brake circuit of the driven vehicle wheels arranged in the immediate vicinity of the brake fluid reservoir and connected to the latter via an inlet non-return valve, the opening pressure of which is at most 0.5 bar and is preferably between 0.2 bar and 0.3 bar. The precharging pump provided in accordance therewith can be accommodated directly in the brake fluid reservoir.
By having the a ASR control valve designed as a 2/2-way solenoid valve, in the neutral position 0 of which the pressure output of the brake unit, allocated to the brake circuit of the driven vehicle wheels, is connected to the main brake line of this brake circuit and with the brake line branching towards the wheel brakes; and in the excited position of which the main brake line is shut off from the brake unit while the pressure output of the high-pressure pump is connected to the main brake line of the brake circuit allocated to the driven vehicle wheels, provides for a particularly simple and functionally reliable configuration of the ASR control valve.
Having the electronic control unit means generate an output signal for setting the ASR control valve to its excited position effecting the shutting off of the brake unit from the main brake line of the rear brake circuit of the driven vehicle wheels, generating on output signal affecting the activation of the high-pressure pump of this brake circuit as well as output signals setting the inlet valves of he wheel brakes of the driven vehicle wheels to their blocking position, as early as when the wheel slip of at least one of the driven vehicle wheels at least reaches or exceeds a predetermined threshold value, the magnitude of which is between the theoretical value of the wheel slip and the response threshold of the wheel slip control, allows the high pressure pump assembly and control valves of the driven wheels to act in a particular sensitive and rapid response to the ASR control.
An additional improvement can be obtained when the output signal of the electronic control unit which effects the activation of the pump drive of the nigh-pressure pump of the driven brake circuit, is generated at least as early as when the wheel slip of at least one of the driven vehicle wheels reaches, or exceeds a pre-determined threshold value, or a value between 50% and 70% of that threshold value, and wherein the ASR control valve means and the inlet valves of the rear wheel brakes are set to their blocking position.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.